1. Field of the Invention
This invention relates to micronized zirconia and to a method for the production thereof. More particularly, this invention relates to a zirconia type micronized particle having an average particle diameter of not more than 20 nm.
2. Description of the Prior Art
Zirconia occurs in three crystal structures, i.e. monoclinic system, tetragonal system, and cubic system. Since zirconia exhibits characteristic properties such as resistance to corrosion, toughness, and oxygen ion-conducting property invariably at levels not found in other materials, it has found utility in variety of applications such as oxygen sensors, electronic parts, and refractory materials. As a material of high performance, zirconia is expected to attract growing respect.
A method for producing micronized zirconia by treating a mixed aqueous solution of a zirconium salt and a salt of a stabilizing agent with an alkaline substance such as ammonia thereby causing coprecipitation of hydroxides of elements involved and calcining the hydroxides has been widely known to the art. The conventional coprecipitation method disclosed as in Japanese Patent Publication No. SHO 54(1979)-23,365, Japanese Patent Publication No. SHO 57(1979)-25,523, or Japanese Patent Laid-open No. SHO 57(1982)-191,234 effects the relevant reactions batchwise. It comprises in adding aqua ammonia to a mixed aqueous solution of a zirconium salt and a salt of a stabilizing element until the pH of the solution reaches a prescribed level thereby causing precipitation of hydroxides. This method for batchwise production of micronized zirconia inevitably entails the following disadvantages.
(i) When the aqueous solution of such a zirconium salt as zirconium oxychloride or zirconium nitrate is neutralized with an alkali such as aqua ammonia to produce zirconium hydroxide through the ensuing phenomenon of precipitation, the resultant micronized zirconia has its particle diameter varied with the pH of the solution during the reaction. When the aforementioned neutralization and the attendant precipitation are effected batchwise, the pH of the solution widely varies when the reaction is started and when it is completed and the produced micronized zirconium hydroxide depends on the time-course change of the pH of the solution during the reaction. The particle diameter of the powder, therefore, is distributed over a wide range.
(ii) In the neutralization-coprecipitation reaction which comprises adding a water-soluble salt of such a stabilizing element as yttrium, calcium, or magnesium to an aqueous solution of such a zirconium salt as zirconium oxychloride or zirconium nitrate thereby producing a mixed aqueous solution and then allowing an alkali such as aqua ammonia to react upon the mixed aqueous solution thereby causing coprecipitation of hydroxides in the mixed aqueous solution, the different components in the solution are precipitated in different pH ranges. Where the pH of the solution widely varies when the reaction is started and when it is completed as in the conventional batchwise reaction system, therefore, the hydroxides of different components are fractionally precipitated. The precipitate eventually obtained, accordingly, possesses a heterogeneous composition.
Zirconium ions are completely precipitated even when the pH of the solution is in the acidic zone, whereas yttrium ions or calcium ions or magnesium ions are not completely precipitated until the pH of the solution becomes alkaline. When the hydroxides are sequentially precipitated by gradually adding aqua ammonia to the aforementioned mixed aqueous solution, which is acidic in nature, thereby varying the pH of the solution in the range of from 8 to 12 in accordance with the conventional batchwise reaction system, therefore, zirconium hydroxide is first precipitated and the hydroxide of the stabilizer is subsequently precipitated. The mixing of these two hydroxides is effected exclusively by agitation. As a natural consequence, the composition has insufficient homogeneity. There are times when the micronized composition, because of insufficient homogeneity, is required to undergo an extra treatment such as wet mixing.
An object of this invention, therefore, is to provide a novel method for the production of micronized zirconia.
Another object of this invention is to provide a method for the production of micronized zirconia of uniform composition and particle diameter.
Yet another object of this invention is to provide a zirconia-containing micronized particle having an average particle diameter of not more than 20 nm.